The present invention concerns computer networks and pertains particularly to runtime detection of network loops.
A local area network (LAN) segment can include repeaters, end stations, and network cabling. For example, the Ethernet protocol operates using a carrier sense, multiple-access with collision detect (CSMACD) protocol by which end stations (e.g., workstations or other computers) first listen for carrier and wait for the network to become idle, then transmit the data, retransmitting the data if collisions are detected. Data is transferred in the form of packets, which contain the address of the sending station (the source address), the address of the intended recipient (the destination address) and a Cyclic Redundancy Check (CRC) of the packet. For more information on the Ethernet protocol, see the standard ISO/IEC 8802-3.
For higher bandwidth networks, multiple network segments are often used. These network segments can be interconnected through the use of one or more bridges or switches. The core functionality of switches and bridges are similar and the terms are often used interchangeably. Bridges contain two or more ports, each of which connect to an network segment. When two stations on different network segments communicate with each other, the bridge will forward the packets between the two segments. When the stations are on the same segment, the bridge will not forward the packets to any other segment.
Bridges learn on which port each station is connected by examining the source address of packets as the packets are received, and storing this address-port association in a filtering database. Bridges operate by examining the destination address of packets received on a given port, and forwarding the packet out the port on which the destination station is connected. If the destination station is on the port from which the packet was received, the packet is not forwarded (i. e., is filtered). For more information on bridges see the standard ISO/IEC 10038.
Network loops can be formed in many ways. For example, when two or more ports on a bridge become connected to the same network segment, a network loop exists. While in this configuration, all packets forwarded by the bridge to any one of its ports on a network segment will be received by all of the other ports that the bridge has on that network segment. The bridge will continue to re-forward and re-receive these packets, the rate and duration of which is bounded only by the limitations of the bridge and the network. Network loops can also be formed, for example, by two repeaters being improperly connected together. Regardless of how a network loop is formed, the result is wasted network bandwidth and wasted bridge resources.
The Spanning Tree Algorithm is the current method most often used to protect user networks from network bridge loops. The Spanning Tree Algorithm defines an algorithm and a protocol that network bridges can use to intercommunicate and then adjust their configurations such that a network is spanned, but does not cause network loops. For more information on the Spanning Tree algorithm, see the standard ISO/IEC 10038.
The Spanning Tree Algorithm adds additional traffic to the network and can only be implemented on network bridges/switches. Further, to be effective the Spanning Tree algorithm must be implemented in all of the network bridges in the network. This is due to the Spanning Tree algorithm not offering protection against network loops that exist on bridges that do not support the Spanning Tree Algorithm (i.e. many unmanaged bridges/switches). This limitation makes it impractical to incrementally implement the Spanning Tree Algorithm on an existing network that contains bridges that do not already support the Spanning Tree Algorithm.
Additionally, when a network administrator makes network connections within a network closet which results in an immediate and heavy increase in network traffic, as indicated by the activity/utilization light emitting diodes (LEDs) for the network, this indicates to the network administrator that potentially a network loop has been formed.